Feedback circuit



April 8, 1941. E. HULZLER FEEDBACK CIRCUIT Filed Aug. 4;, 1958 Ill HYVENTOR F/{W/N HOLZLE Q AT I'ORNEY Patented Apr. 8, 1941 T OFFICE FEEDBACK CIRCUIT Erwin Hiilzler, B'erlin-Halensee, Germany, as-

signor to Fides Gesellschaft fiirj die Verwaltung und Verwertung von .gewerbliche n Schutzrechten mit beschrankter Haftung, Berlin, Germany, a corporation of Germany Application Auguste, weasel-in No. 222,971 1 I i In Germany August 7 1937 v 4 Claims. (Cl. 179-471) In amplifiers with negative feedbackinwhich a high degree of feedback is chosen with a'yiew to insuring adequate linearizing action (correc tion of distortion) there exists the risk of positive feedback occurring for certain frequencies as a result of phase rotation producedby the coupling means, and this would impair the stability of the amplifier. In order to obviate these difficulties, attempts have been made to so form and dispose the coupling means as well as the circuit elements included in the feedback or regenerative path that the maximum phase rotations will arise at frequencies, especially those outside the range of the useful or signal wave, where little disturbance may be anticipated as a result of such inadvertent, positive feedback, or for which the positive feedback is suppressed by distinct means provided to that end.

However, all of the means disclosed in the art, as far as I am aware, have failed to insure an all-around satisfactory solution. In the majority of cases, in order to obtain adequate stability, the degree of the feedback had to be made lower than would be desirable with a view to insuring the intended correction or linearizing effect.

Now, according to the invention, to the end of insuring complete preclusion of undesired phase shift or rotation, circuit elements are used in the feedback path which are so composed of positive and negative resistances that the resultant phase shiftwill be zero. For a further explanation of these circuit organizations reference is here made to the embodiments shown by way of example in the appended drawing in which Figure 1 shows a circuit arrangement for obtainf ing a negative inductance and its equivalent inductive value; Figure 2 shows a feedback net work employed in the present invention and Figure 3 shows an application of the present invention to an amplifier.

Figure 1 shows a T-type network terminating in a condenser C, the said organization comprising the two positive ohmic series resistances R and the negative ohmic shunt (or cross-arm) resistance (R). This circuit arrangement is equivalent to a negative inductance (-L) having a value C R The negative resistance (--R) may consist of a tube in a dynatron circuit arrangement or of any suitable conductor having a negative temperature coefficient and which is appreciably heated by the current flow therethrough from a cooperating power source. A suitable material for constructing such a conductor is uranium dioxide.

Now, if this negative inductance, in a way as shown in Figure 2, is connected together with function of the, frequency, but of real value, and

presenting zero phase angle. The size of the said impedanc is A R-w R1 Now, if circuit organizations of this kind are included in the feedback path of an amplifier with negative regeneration it is possible to raise the degree of feedback to any point which may be requisite to obtain the desired correction of dis tortion (linearization), inasmuch as the chance of positive feedback, owing to the absence of any phase rotation at all, is perfectly precluded. There is no need to make all of the coupling elements included in the feedback path of circuit organizations of the said sort, always provided that-the residual elements of standard design present a phase rotation which will not exceed the permissible limit. An organization of this type is shown in Figure 3 in which the reference numeral 5 indicates an amplifier of any conventional form of construction in which it is desired to use negative feedback in order to compensate for frequency distortion. The gain of the amplifier without the feedback is indicated by the character a. From the output 6 of the amplifier to the input 1 is connected a feedback network 8 which is constructed as shown in Figure 2. This feedback network comprises a pair of serially connected positive inductances L and L and a shunt connected negative inductance L. Across the output of the T-type network is connected a load resistance R1 across which the final feedback voltage which is applied to the input of the amplifier appears. The negative inductance L is constituted byresistances and capacities connected in the way shown in Figure 1.

Since there is no difliculty in securing any desired frequency response for the circuit organization here disclosed, it will be possible to accomplish simultaneously by the same manner and means the desired correction of distortion with the amplifier.

The invention is not confined to the circuit arrangements hereinbefore described by reference to the appended drawing showing exemplified embodiments, but in fact, it encompasses equivalent circuit organizations as well as circuits in which, owing to the simultaneous use of positive and of negative resistances, the phase rotation angle is diminishable to the zero level.

I claim:

1. An amplifier having an input and an output, means for feeding back energy from said output to said input including a correcting network having an impedance which varies in a predetermined manner with the frequency applied thereto, said network comprising series positive inductances and a negative shunt inductance, said inductances being so proportioned that no phase shift is introduced.

2. An amplifier having an input and an output, means for feeding back energy from said output to said input including a correcting network having an impedance which varies in a predetermined manner with the frequency applied thereto, said network comprising a pair of serially connected series positive inductances and a negative shunt inductance, said inductances being so proportioned that no phase shift is introduced.

3. In an amplifier for oscillatory energy of a variable frequency and having an input and an output, a channel for feeding back energy from said output to said input, said channel having a resistance oi! a real value of a magnitude which is a function of the frequency of the oscillatory energy applied thereto and comprising serially connected positive impedances and a negative shunt impedance so proportioned with respect to said positive impedances that the energy at both ends of said channel has an in-phase relationship over the range of said variable frequencies.

' 4. In an amplifier for oscillatory energy of a variable frequency and having an input and an output, a channel for feeding back energy from said output to said input, said channel having a resistance of a real value of a magnitude which is a function of the frequency of the oscillatory energy applied thereto and comprising serially connected positive inductances and a negative shunt inductance so proportioned with respect to said positive inductances that the energy at both ends of said channel has an in-phase relationship over the range of said variable frequencies.

ERWIN H6LZLER. 

